With the development of mobile communication technology, a mobile communication network changes every generation. Currently, a network adopts a structure in which a synchronous mobile communication system (Code Division-Multiple Access [CDMA] mobile communication system), designated as second generation or 2.5-th generation network, and an asynchronous mobile communication system (Wideband CDMA [WCDMA] mobile communication system), designated as a third generation network, coexist.
Further, in order to support global roaming between mobile communication systems, a Dual-Band Dual-Mode (DBDM) mobile communication terminal capable of being used in both a synchronous mode system and an asynchronous mode system has been developed. By using the mobile communication terminal, different types of services can be used in asynchronous and synchronous mode system areas.
Currently, an asynchronous mobile communication system is being constructed in areas having a great number of service requests, so that a synchronous mode mobile communication system has been developed into a form in which the service area thereof includes service areas of an asynchronous mode system. However, since the asynchronous mobile communication system is still in an initial stage and requires enormous investment to be implemented, the service cannot be provided to a wide area, so that the service area of an asynchronous mobile communication system overlaps with that of a synchronous mobile communication system.
Accordingly, since the service area of the asynchronous mobile communication system is limited, there is a problem in that service is interrupted when a subscriber to the asynchronous mobile communication system moves into a synchronous area, in which an asynchronous mobile communication service cannot be provided, during the use of the packet data service in an asynchronous area.
Further, there is a problem in that service is interrupted when a subscriber to the asynchronous mobile communication system, whose terminal is in a dormant state in an asynchronous area, moves into a synchronous area in which an asynchronous mobile communication service is not provided.
In this case, “dormant state” means a state in which a mobile communication terminal using a packet data service is in a disconnected state when the transmission/reception of data does not occur between the mobile communication terminal and a mobile communication system, and can immediately make a transition to an active state when a data communication signal is generated. If a dormant state is not maintained at the time of handover from the asynchronous mobile communication system to the synchronous mobile communication system, that is, if a packet data service is completely released, there is a disadvantage in that the mobile communication terminal must again perform a procedure of connecting to the synchronous mobile communication system from the beginning in order to use the packet data service, thus causing the deterioration of service quality, such as the increase of a connection time.
As described above, if asynchronous and synchronous mobile communication systems coexist and the service area of the asynchronous mobile communication system is smaller than that of the synchronous mobile communication system, handover is required to provide continuous packet data service between the asynchronous and synchronous mobile communication systems.